A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
When a pattern is transferred onto a target portion via imaging, this is usually done using a projection system that will be discussed in more detail below. In order to obtain a projection of high quality, the substrate should be positioned accurately with respect to the projection system, i.e. in a focal plane of the projection system, taking into account the local shape of the substrate. Measuring the shape of a substrate and positioning the substrate with respect to the projection system is called leveling.
To level accurately, a level sensor may be used to measure the shape of the substrate, based on which during exposure, the position and orientation of the substrate may be adjusted to achieve optimal imaging results. The level sensor measurements may be performed before start of the exposure, for instance in a multi stage lithographic apparatus. The level sensor measurements may also be performed during exposure (on the fly), for instance in a single stage lithographic apparatus.
The level sensor may perform height measurements to generate height data.
However, level sensor measurements may fail when measuring near the edge of a substrate, for instance because all or part of the measurement beams of the level sensor fall outside the substrate or fall within an edge area of the substrate in which no valid measurements may be obtained.
According to the prior art, leveling in the areas where no valid level sensor measurements are obtainable may be done by using height data from level sensor measurements from nearby areas, for instance by extrapolation of level sensor measurements from nearby areas. However, using information from nearby areas (typically not areas along the edge of the substrate) is not very reliable, as the shape of the substrate may be deviating near the edge with respect to inner areas.
According to an alternative solution described in US 2005-0134865 A1, the global shape of the substrate near the edge of the substrate is determined based on a so-called global level contour, describing the average ‘shape’ of the substrate near the edge. Such a global level contour (GLC) may be determined by performing a special measurement, in which the level sensor is used to scan along the edge of the substrate and typically comprises three parameters: Rx, Ry (rotation about x and y axis respectively) and Z (the z-axis substantially perpendicular with respect to the surface of the substrate the pattern is to be applied to, and the x- and y-axes substantially perpendicular with respect to the z-axis and with respect to each other). This causes defocus on the edge field since the GLC-based height and tilt is often too far away from the actual local height and tilt. The defocus is often that large (up to a few hundred nm) that all dies in the field become non-yielding and thus unusable. Also, lines in poorly imaged dies may fall over and may be a contamination source for next process steps.